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When interpreting whether an image is normal or abnormal, it is common to come across incidental lytic lesions, which, depending on their appearance, must be classified as either a normal variant, or something which warrants further investigation.

It is difficult to determine radiologically with plain radiograph imaging if a lytic lesion is benign or malignant. It is more accurate to describe whether the process looks aggressive or non-aggressive. Some factors, as outlined below, help to determine whether a lesion looks aggressive or non-aggressive, and therefore the differential diagnosis.

It is important to remember, however, that some benign processes such as osteomyelitis, can mimic malignant tumours, and some malignant lesions, such as metastases or myeloma, can look benign.

Factors aiding in the diagnosis of bone tumours and benign lytic lesions:

Age of patient

Specific lesions tend to occur in specific age ranges. Solitary bone cysts, non-ossifying fibromas, aneurysmal bone cysts and Ewings tumours occur in patients under the age of 30 years. Metastases and myeloma will usually occur in patients over the age of 40

Location within the bone

Epiphyseal, metaphyseal or diaphyseal

Central within the bone, eccentric or cortical

Lesions often arise within specific bones, and within specific areas of that bone. Giant cell tumours for example, usually arise within the distal femur or proximal tibia, and will always abut (push against) the articular surface

Size of lesion

Size of lesion is not necessarily an indication of how aggressive the process is, but recognition that specific lesions have a tendency to grow larger can help lead to the correct diagnosis. Solitary bone cysts within the proximal humerus, for example, often become large. A large lytic lesion is at risk of fracturing and it is therefore often prophylactically packed to prevent fracture and subsequent deformity

Monostotic (one lesion) or polyostotic (multiple lesions)

Multiple lesions are also not necessarily indicative of an aggressive process. Although metastases and myeloma are usually multiple, most aggressive processes demonstrate a single lesion. Similarly, benign enchondromas often become multiple within the phalanges

Zone of transition from normal to abnormal bone

This is often the best indicator as to whether a lesion is aggressive or non-aggressive. A very definite, sharp, and therefore narrow area (zone) between the normal and abnormal bone indicates a non-aggressive lesion. A wide, hazy, and undefined zone of transition suggests a more aggressive process. However, be aware that some benign processes (osteomyelitis) have a wide zone of transition as they are fast acting

Reactive sclerosis

If there is a sclerotic margin to the lesion, it is most likely non-aggressive

Pattern of bone destruction

Presence of visible tumour matrix

Cartilage = Chondroid calcifications

Osteoid = Sclerotic

Fibrous = "Ground glass", hazy opacification

Host (bone) response

Cortical thinning, expansion and penetration. Cortical destruction suggests an aggressive process. Be aware, however, that what may appear to be cortical destruction may actually be cortical bone replacement by a fibrous or chondroid matrix, which is non-calcified and may be located within a benign lesion. This gives the false impression of cortical destruction when it is actually cortical replacement. Aneurysmal bone cysts, for example, often cause such thinning of the cortex as to make it undetectable radiographically

Periosteal reaction

Periosteal reaction will occur whenever the periosteum is irritated. This may be due to a malignant process, a benign lytic lesion, osteomyelitis, or trauma. The appearance of the periostitis will give an indication as to cause:

Benign periostitis looks thick, wavy, dense and uniform, as it is slow growing and therefore gives the periosteum time to lay down new bone

Aggressive periostitis is often described as lamellated (onion-skinned), amorphous and sunburst as the periosteum does not have time to consolidate

Soft tissue involvement

Aggressive lesions often lead to cortical breakthrough to create a soft tissue mass

BENIGN LYTIC LESIONS

Non-Ossifying Fibroma / Fibrous Cortical Defect

One of the most common benign lytic lesions seen

Asymptomatic and usually an incidental finding

Most often seen around the knee and distal tibia

Non-Ossifying fibroma generally bigger than 2cm

Fibrous Cortical Defect generally smaller than 2cm

Arises in the under 30 year age group

Develops from cortex of metaphysis; is eccentric within the bone

Usually has thin, sclerotic border that is often scalloped and slightly expansile

Become sclerotic as healing occurs and "disappears" as it ossifies

Therefore not seen in the over 30 age group

Simple / Solitary Bone Cyst

Arises in the under 30 year age group

Begins within the physeal growth plate and extends into diaphysis

Centrally located within a long bone

Most commonly occurs in the proximal humerus

In the calcaneum it is triangular, and located antero-inferiorly as this is an area that does not receive stress, and therefore develops atrophy of the bony trabeculae

Also called unicameral bone cyst, however there is not always just one compartment

Asymptomatic, unless it is fractured, which often occurs

"Falling fragment sign": cortical fragments produced from pathological fracture, that have sunk to the bottom of the fluid filled lesion

Aneurysmal Bone Cyst

Arises in the under 30 year age group

Presents with pain and swelling

Expansile lytic lesion with a thin sclerotic margin

Eccentrically located in the metaphysis of a long bone, adjacent to the unfused physeal growth plate